The best biological correlate of learning and memory is synaptic plasticity, or changes in the number and strength of synapses with experience. Long-term synaptic plasticity is a transcription- and translation-dependent process that can be restricted to subsets of synapses within a single neuron. We propose that one mechanism for restricting gene expression to individual synapses is through mRNA localization and regulated translation. The model system Aplysia californica provides an excellent model system to study synaptic plasticity. Previous work in the Martin lab has demonstrated that protein synthesis can be spatially restricted to stimulated synapses during learning- related plasticity of cultured Aplysia sensory-motor neurons (Wang et al., 2009). In this proposal, I focus on the mechanisms underlying the synaptic localization of the mRNA encoding an Aplysia sensory cell-specific neuropeptide, sensorin. Studies in neurons and other asymmetric cell types have shown that localized transcripts often contain cis- acting localization elements in their untranslated regions (UTRs), which often are encoded by stem-loop structures. The Martin lab has demonstrated that the 5' and 3 untranslated regions (UTRs) of sensorin are sufficient for synaptic localization of reporter RNA. While the 3'UTR is required for distal neurite localization, the 5'UTR is required for synaptic localization. I have identified a 66 nucleotide sequence in the 5'UTR of sensorin mRNA that when paired with the sensorin 3'UTR is required and sufficient for sensorin mRNA synaptic localization. My experiments further indicate that this localization element is encoded by a stem-loop structure (Meer et al., 2012). Cis-acting RNA elements interact with trans-acting RNA binding proteins to mediate RNA localization. In addition to characterizing the neuritic RNA localization element in the sensorin 3'UTR, I propose experiments to determine how cis-acting elements function in the RNA stability and translation of sensorin. Finally, I will identify the RN binding proteins that localize sensorin mRNA to neurites and synapses and study the dynamics of these proteins during synapse formation. The results of the proposed experiments will provide insight into the cell and molecular biological mechanisms by which neurons are able to spatially restrict gene expression through mRNA localization and regulated translation. They are of significance to a range of brain disorders in which long-term synaptic plasticity is impaired, including autism, mental retardation, anxiety disorders and drug addiction.